Just replaced the seal in my Original mod 5and was as shown in the chambers parts list. My thoughts are that it is a modification done by a previous owner or a gunsmith as this seems to be a common thing to do to if you have a lathe - will takes about 15 mins but leaves the next owner with a problem of finding the right O ring. My suggestion is to look online as there a engineering firms with good websights. I often do this when faced by simmilar situations like you. You can even but a length of the rubber of the right Dia and super glue it to the correct size but for a moving seal I would not do this but find the correct one. There are lots of different materials too. Bear in mind that the one you have may have been squashed and will look thicker or may have swollen due to being oiled.
Good luck let us know how you get on as the mod 5 is a great little pistol
George